1. Field of the Invention
The present invention relates to an injection molding method for manufacturing a functional part having a recess, e.g., a borehole for a hydraulic functional part of a motor vehicle.
2. Description of the Related Art
Units having hydraulic functions for the automotive sector, such as transmission hydraulic modules, transmission valve plates, ABS hydraulic units, and brake master cylinders, are primarily made of metals such as aluminum and steel due to the required precision of the long boreholes, and pistons and valves that are moved therein, and are made of plastics, for example duroplastics or thermoplastics, only in some areas. The component geometries necessary for the function have ribs, different wall thicknesses, and transverse holes which may result in warping, sink marks, and joint lines during filling with plastic molding compounds.
Although duroplastics have advantages over thermoplastics with respect to precision and dimensional stability over a wide temperature range, as well as with regard to swelling behavior in the operating materials, they exhibit severe burr formation in injection molding processing. After production, these burrs must be removed by blasting processes. This is difficult to ensure for larger parts having deeply recessed borehole cuttings, and results in soiling problems. The curing of the duroplastics required in the process prolongs the cycle time, and necessitates accurate testing of the curing behavior of the molding compound prior to processing, as well as specialized process control of the curing behavior. In addition, duroplastics have a higher density than thermoplastics.
Thermoplastics offer advantages with respect to freedom from burrs, more uniform and quicker processing, and a lower part weight.
However, because of the known disadvantages of thermoplastics, such as poorer dimensional stability, greater anisotropy in fiber reinforcement, swelling in operating materials such as brake fluid, transmission fluid, and automatic transmission fluid (ATF), as well as sink marks and geometric deviations of joint lines compared to metals and duroplastics, thermoplastic housings having hydraulically functional boreholes are usually not used.
The published German patent application document DE 60 2004 008 858 T2 describes a hydraulic unit which is manufactured by injection molding of a resin material, reinforcing elements being initially produced by injection molding, and being subsequently extrusion-coated with resin in a further injection molding step for manufacturing the housing. However, there is a risk that the reinforcing elements may be deformed by the large quantity of resin which is injected at high temperatures and pressures for manufacturing the housing.